The securing function of intramedullary nails is already known in the state of the art. The locking screws or locking pins (hereinafter only the term “locking screw” is used, but this is also intended to include the term “locking pins”) are inserted into the transverse bores in an intramedullary nail either with the help of an imaging method (x-ray monitoring) or a more or less complex target device. In both cases, a certain target inaccuracy is unavoidable, i.e., the tip of the screw cannot be precisely aligned coaxially with the central axis of the transverse bore but instead deviates therefrom by a certain amount. In order for the locking screw to open into the transverse bore and in order for it to be passed through this bore despite this target error, the outside diameter of the screw is undersized in relation to the diameter of the transverse bore. If the target inaccuracy remains within the scope of this undersizing, then the locking screw can be passed through the transverse bore with no problem despite this target error. However, the locking screw has a certain play in relation to the transverse bore, because of the undersizing.
This play defines the amount by which the main bone fragments, which are to be secured in the corresponding locking hole by means of locking screws, are able to move in relation to the nail and thus also, owing to the rigidity of the nail, in relation to other main bone fragments secured using the same nail. To ensure the applicability of this locking means for the surgeon, this play is unavoidable but nevertheless undesirable clinically for certain indications (e.g., in the case of metaphyseal fragments).
Even nails having a solid cross section, which may have an inside thread in the locking hole, are not free of play. The inside thread only prevents the nail from being axially displaceable on the locking screw.
U.S. Pat. No. 6,296,645 HOVER ET AL. describes a hollow intramedullary nail made of metal having one or two plastic inserts in the jacket openings in the transverse bore that are positioned diametrically opposite one another and are referred to as windows, the locking screw being insertable through these plastic inserts. One disadvantage of this known intramedullary nail is the fact that the window-like plastic inserts can easily be pushed in, which causes the desired function to be lost. However, even with very cautious manipulation, the two plastic inserts could be forced out of their “window” in pushing the locking screw through, which would also lead to a loss of function.